What does hypotonic mean in terms of solution concentration?

Hypotonic refers to a solution that has a lower concentration of solute compared to another solution. In biological contexts, this term is often used when comparing the concentrations of solutes inside a cell and in the surrounding fluid. When a solution is hypotonic to the cytoplasm of a cell, it means that there is a lower concentration of solute (such as salts or sugars) outside the cell than inside. This difference causes water to move into the cell by osmosis, as water moves from an area of lower solute concentration to an area of higher solute concentration to achieve equilibrium. This movement can lead to cell swelling and potentially lysis if the difference is significant enough.

Other options do not accurately define hypotonicity. A higher concentration of solute describes a hypertonic solution, where water would move out of the cell. Equal concentrations describe an isotonic solution, where there is no net movement of water across the cell membrane. Finally, a solution containing no solutes at all would be pure water, which is not applicable in the context of hypotonic solutions since hypotonicity necessarily involves comparison to another solution with a higher concentration.